Multi-Tech Systems BL-Series - Manual (Page 159)

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Appendix J - RS-232C Interface Specifications
External Transmit Clock - Pin 24, XTC
Direction: to modem
Supplies the same function as transmit clock on Pin 15.
Terminal Busy (Out of Service) - Pin 25, OOS
Direction: to modem
The Terminal Busy (OOS) circuit is not defined in the RS-232C standard, but is used by us and most
manufacturers to make the phone lines busy to incoming phone calls. Pin 25 is not normally
connected to the RS-232C interface when shipped from the factory; it can be ordered connected as
an option. If pin 25 is connected and is brought high, the modem will be placed Off Hook, and busy to
incoming calls.